1. Field of the Invention
The nutritional value of milk is associated predominantly with its protein and fat contents. While historically pricing has been based only upon the level of fat, increasing emphasis is now being placed on the percentage of protein as well. Accordingly, milk producers, dairy cattle breeders, and organizations such as the Dairy Herd Improvement Association (DHIA) require a quick and reliable method for determining the protein and fat contents of milk. The method should be applicable to assaying bulk lots as well as to samples collected daily from individual cows. For the on-site monitoring of individuals and small test groups, economics dictate a relatively simple procedure employing inexpensive and compact instrumentation. This invention relates to an automated assay method and apparatus designed specifically for this purpose.
2. Description of the Prior Art
An early batch-type photometric procedure for the determination of fat content in milk is taught by Borg in U.S. Pat. No. 2,844,067. A beam from a filtered light source is passed through emulsified samples and detected by a photoelectric cell. The percent fat is displayed on a meter as a function of the cell's output voltage. Calibration of the system is accomplished by regulation of the lamp intensity.
In U.S. Pat. No. 3,960,493, Beitz et al. provides a nephelometric method for the sequential determination of protein and fat in a milk sample. By addition of selective reagents, the protein and fat are alternately converted to colloidal dispersions, and each measured as a function of turbidity independent of the other.
Huang et al. [J. Food Sci. 41: 1219-1221 (1976)] report on a colorimetric procedure for assaying milk protein based on a modification of the Lowry method. Samples with and without the color-developing reagent are sequentially pumped into a flow-through cuvette, and in each case the absorbance is measured by means of a single-beam spectrophotometer. The difference in the respective absorbances null out the effect of the fat without any provision for determining its level.
While each of these prior art procedures are useful for its intended purpose, none combine the speed, accuracy, and versatility of multicomponent analysis presently demanded by many of the research and marketing activities of the dairy industry.